This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No. PCT/JP97/02008 which has an International filing date of Jun. 11, 1997 which designated the United States of America, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a lithium secondary cell. In particular, the present invention pertains to lithium secondary cells suitable as backup batteries for watches, power sources for portable devices such as pagers, timers, etc., backup batteries for memories, and the like.
2. Description of the Related Art
Lithium secondary cells comprising a negative electrode made of metal lithium or a lithium alloy have been mainly developed as secondary cells. However, when the metal lithium or lithium alloy is contained in the negative electrode, lithium ions in an electrolyte tend to precipitate in the form of metal lithium on the negative electrode during recharging. The deposited lithium forms minute particles or grows lithium dendrites on the surface of the negative electrode, and causes a short-circuit in the cell. Thus, the charge-discharge cycle life of the cell is shortened. Consequently, lithium cells, which use neither metal lithium nor a lithium alloy in the negative electrode and have a high energy density, have been studied.
Currently, primary cells such as silver oxide cells are used as power sources for wristwatches. However, the primary cells suffer from problems associated with disposal of the used cells. Thus, wristwatches having built-in power generators, which require no replacement of the cells, have been developed, and electric double layer capacitors are used as power sources used in such wristwatches. However, the electric double layer capacitors have a small capacity per unit volume, and thus it is desired to develop substitute power sources for the electric double layer capacitor.
One object of the present invention is to provide a lithium secondary cell which can be charged and discharged, does not suffer from the problem of disposal, and has a high capacity so that it is suitable as a power source for a wristwatch.
Accordingly, the present invention provides a lithium secondary cell comprising a positive electrode containing lithium titanate as an active material, a negative electrode containing a carbonaceous material as an active material, and an electrolytic solution comprising a solution of a lithium salt in an organic solvent.
Since a lithium titanate as a positive electrode active material and a carbonaceous material as a negative electrode active material are used in combination, the lithium ions can be easily doped and dedoped at a nominal voltage of 1.5 V, and thus a lithium secondary cell having a high capacity and good charge-discharge cycle properties is obtained.